Biomedical Engineering Reference
In-Depth Information
z
x
b
a
b < a
MM256
y
Modal birefringence B = (β x y ) λ ° /2π
Beat length L = λ/ B
FIGURE 4.10
Optical fiber with elliptical cross section.
occur as well, particularly with increasing thickness of the jacket material.
Sensitivity to static pressure changes is low compared to other effects; how-
ever, longitudinal strains due to acoustic signals can provide a pronounced
perturbation of the system signal. Stabilization techniques for these effects
are described in a later section.
Another fiber consideration arises from the fact that the coherent detection
process used in the fiber interferometer requires that the polarization at the
output of both fibers remains the same. This can be solved most efficiently
with polarization-maintaining fibers. When the fiber is wound on a drum
or mandrel, linear polarization-maintaining elliptical fibers as illustrated in
Figure 4.10 are preferable to the circular polarization-maintaining fibers.
4.7 FiberTransducerConsiderations
Transducer design for the fiber magnetometer is again representative of many
necessary considerations for other fiber sensor systems, such as fiber pressure
and acoustic sensors. As mentioned previously, two approaches can be con-
sidered in the coupling between the magnetic material and the fiber. A coat-
ing of magnetic material can be applied directly on the fiber, or the fiber can
be attached by an adhesive to an already formed heat-treated magnetic strip
or film. Each approach requires a unique set of processing considerations.
A number of coating techniques are available. These include vapor deposi-
tion, sputtering, electrodeposition, and electroless deposition. Each has been
used successfully in the coating of recording discs for information storage
[8]. The application of magnetostrictive active magnetic coating to an opti-
cal fiber is a difficult process. Both sputtering and vapor deposition are very
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